Voltage comparator circuits are commonly used building blocks in electronic systems. A voltage comparator circuit provides an indication of whether one input voltage is higher or lower than a second input voltage. Today's portable electronic devices require low voltage, low power voltage comparator circuits for such functional blocks as analog to digital converters, low battery detection circuits, and the like.
A typical 2 volt comparator is discussed in the book Delta-Sigma Data Converters--Theory, Design and Simulation edited by Steven Norsworthy, Richard Schreier, and Gabor Temes of the IEEE Press 1997, and is illustrated on page 237. This comparator, exemplifying the typical circuitry utilized in today's communications circuits, comprises two cross-coupled latches and two switches.
The drawback of this circuit is that it operates at a minimum supply voltage of 2 volts, whereas low power portable systems today are moving to lower supply voltages.
Low voltage comparator circuits capable of 1 volt operation are being developed to meet the needs of these low power portable systems. One such design is discussed in an article entitled "A 900 MV 40 .mu.W Switched Op amp Delta-Sigma Modulator with 77 dB Dynamic Range" by Vincenzo Peluso, et al., 1998 IEEE International Solid-State Circuits Conference, page 68. This article describes a circuit including a differential amplifier and a cross-coupled latch, with reset switches in parallel with the latch. The inputs are fed into a PMOS differential pair, limiting the input common-mode signal to the range GND to 0.25V. This reduced input operating range is a significant limitation over the range of operation of conventional 2 volt comparators, which usually have an input common-mode range of 1V. Thus, it would be preferable for a 1V comparator circuit to have a rail-rail (supply to ground) input common-mode operating range to have comparable performance to the 2V comparators.
What is needed, therefore, is a low voltage comparator capable of operating at a low supply voltage such as 1 volt, and over a wide input voltage range, for example from the supply voltage down to ground.